Conventionally, a silicon crystal solar cell is well known as a method of directly converting optical energy to electric energy and it has been widely used in the area of consuming feeble power and for standalone power supply and space power supply. However, since enormous energy is required for producing amorphous silicon as well as silicon single crystal, it is necessary to continue electric power generation for a long time period of nearly ten years in order to recover energy consumed for manufacturing a battery.
In such a situation, a dye-sensitized solar cell using a dye has received widespread attention. This dye-sensitized solar cell is predominantly composed of a semiconductor porous electrode, carrying a sensitizing dye, formed on a transparent conductive layer on a transparent substrate, a counter electrode and a carrier transport layer sandwiched between these electrodes and expected as a next-generation solar cell because of the ease of a fabrication method and low material cost.
In J. Am. Ceram. Soc., 80 (12), 1997, p3157-3171, there is described a fabrication method of a dye-sensitized solar cell in which a sensitizing dye such as transition metal complex is adsorbed on the surface of titanium oxide. In this method, by immersing the titanium oxide porous electrode formed in a transparent conductive layer on a transparent substrate in a solvent in which a sensitizing dye is dissolved, the porous electrode carries the sensitizing dye. Then, an electrolyte containing a redox system is added dropwise and a solar cell is prepared by overlaying the counter electrode on the porous electrode.
In this solar cell, when visible light is irradiated to a photoelectrode, the sensitizing dye on the surface of the semiconductor absorbs light, and thereby, an electron in a dye molecule is excited and an excited electron is injected into the photoelectrode. Thus, an electron is generated on this electrode side and this electron moves to the counter electrode through an electric circuit. The electron transferred to the counter electrode is transported by a hole or an ion in the carrier transport layer and returned back to the photoelectrode. A process like this is repeated and electric energy is drawn out to realize high energy conversion efficiency. However, further improvement in conversion efficiency is essential in order to commercialize this method as a solar cell, and therefore increases in a generated current (short-circuit current) and an open-circuit voltage are desired.
For the purpose of increasing the generated current, for example in Japanese Unexamined Patent Publication No. HEI 10 (1998)-255863, there is proposed a dye-sensitized solar cell which is intended to constitute a photoelectrode by providing a layer (layer of light reflecting particle) containing large semiconductor particles having an average particle diameter of, for example, 200 to 500 nm as a constituent material on a plane which is far away from a light receiving plane of a photoelectrode (layer of light absorbing particle) containing small semiconductor particles having an average particle diameter of, for example, 80 nm or less as a constituent material and to improve absorption efficiency of incident light by scattering incident light entering the photoelectrode by the light reflecting particle.
And, in Japanese Unexamined Patent Publication No. 2000-106222, there is proposed a dye-sensitized solar cell which is intended to improve absorption efficiency of incident light by scattering incident light entering the photoelectrode by arranging semiconductor particles (an average particle diameter; 10 to 300 nm) having a large particle diameter together with semiconductor particles (an average particle diameter; 10 nm or less) having a small particle diameter in a photoelectrode.
Further, in Japanese Unexamined Patent Publication No. 2002-222968 and Japanese Unexamined Patent Publication No. 2002-352868, by overlaying a plurality (three or more) of layers, the plurality of layers being different in an average particle diameter of semiconductor particles constituting a semiconductor layer from one another, an optical confinement effect in the semiconductor layer is improved and a generated current is increased.